(1) Field of the Invention
The present invention relates to a solid-state image sensor in which unit pixel cells are arranged in a matrix, each cell including a photo-electric conversion element formed on a semiconductor substrate, and a color filter formed on the photo-electric conversion element. The present invention also relates to a method of manufacturing the solid-state image sensor.
(2) Description of the Related Art
Speaking of solid-state imaging sensors such as a Charge Coupled Device (CCD) sensor and a Metal Oxide Semiconductor (MOS) sensor, shortening of exit pupil distance between a lens of camera and the solid-state image sensor, and the miniaturization of cells are progressing, along with miniaturization of cameras and an increase in the pixel count of solid-state image sensors.
FIG. 1 is a cross-sectional view of the solid-state image sensor disclosed in Japanese Laid-Open Application No. 2001-249218. FIG. 1 shows a cross-sectional view of two unit pixels located particularly in the periphery of an imaging area. In the solid-state image sensor, a flattening film 14 such as a Boro-Phospho Silicated Glass (BPSG) is formed on a photodiode 11 of a photoelectric conversion unit, and an in-layer lens 3 is formed on the flattening film. In order to reduce variation in finishing of each color filter, a transparent film 4 is formed on the in-layer lens 3 for flattening the surface of the in-layer lens 3, while a color filter 2 is formed on the transparent film 4. The color filter 2 can be a color filter made of complementary colors such as yellow, cyan, magenta, and green, or a color filter made of primary colors such as red, green and blue. A micro-lens 1 and the color filter 2 are positioned offset toward the center of the imaging area, in order to increase an amount of oblique light received by the photodiode 11 in the periphery of the imaging area.
Color filter layers have conventionally been formed based on, for example, a dyeing method or a color resist method. In the color resist method, selective exposure and development processing are performed on a photo resist film that contains colorants and pigments, so as to form a color filter of intended color. Then, a transparent flattening film is formed below micro-lens for reducing the unevenness in the surface of the color filter after the formation of the color filter, which results in less variation in the formation of the micro-lens 1.
According to the improved example disclosed in the Japanese Laid-Open Application No. 2001-249218, thin-sliming of the complementary green filter, which is formed by a combination of cyan and yellow and is therefore very likely to be the thickest film among the color filter layers, is suggested. This is because the shortening of the distance between the surface of the semiconductor substrate and the transparent flattening film 13 that is located below the micro-lens is necessary as a solution to the problems such as degradation of luminous sensitivity to oblique light and color mixing caused between neighboring pixels, which are due to miniaturization of cells.
In the method of manufacturing a color filter, a color filter coating, which is a color filter component of the first color and does not contain a photosensitizing agent, is formed by selective etching. Therefore, for example, in the process of manufacturing a complementary color filter, by forming a green filter component using the same method as applied to the first color, it is possible to form a green filter component as thin as a filter component of other colors, which leads to thin-sliming of color filter layer.
However, along with the shortening of exit pupil distance between a lens of a camera and a solid-state image element it has been difficult, with the conventional solid-state image sensor, to reduce the degradation of light sensitivity and the color mixing caused between the neighboring pixels, by simply rendering the color filter thinner. This is because a cross-sectional shape of the color filter is not always finished by vertical cut: the firstly formed color filter has a trapezoidal shape (i.e. the upper base length is shorter than the lower base length) and secondly and thirdly formed color filters are inserted into the space where color filters are not yet formed. As a result, oblique incident light passes through the edge of the color filter of the neighboring pixel, as shown in FIG. 2A, and desired spectral characteristics cannot be obtained. Another problem with the conventional cross-sectional shape is that an alignment margin decreases as the miniaturization of solid-state image sensors progresses. In the case where a color filter and a micro-lens are not properly aligned, color mixing is caused by the fact that the oblique incident light passes the edge of the color filter of the neighboring pixel, as shown in FIG. 2B, and desired spectral characteristics cannot be obtained. Further problems such as color density difference between lines, flicker, color shading, and variation in sensitivity are caused since the degree of color mixing varies depending on the angle at which the incident light passes. The color density difference between lines is a difference in density of color between the neighboring lines caused by the difference in the degree of color mixing between lines due to the different arrangement of colors in each line. The flicker attributes to a spectral difference between fields due to color mixing in the case of complementary color filter. The color shading is a phenomenon that the color differently develops between in the periphery of the imaging area and in the center.
As described above, technology which prevents decrease in light sensitivity that is a main characteristic of the solid-state image sensors as well as other characteristics such as color mixing between the neighboring pixels, a color density difference between lines, flicker and color shading is needed as the miniaturization of solid-state image sensors progress.